That’s just not true.
Takeover Aug 2019:
They also in this time made their first cup final in decades.
Nice had a couple of stand out seasons in 2015 & 2016, before reverting to type for a couple of seasons prior to the takeover, anyone pretending those two seasons were the norm needs their heads checking.
So Nice absolutely have matched what INEOS took over and after two 5th place finishes & a cup final they now have a highly rated DoF at the helm for the next step in their long term vision.
Nice fans not being happy is due to them being guilty as many fans and expecting the takeover to bring them instant Chelsea, City, PSG esque success.
Lets be a little more honest...
The 4 seasons before the take over they finished 4th, 3rd 8th and 7th
The 4 seasons after the take over they finished 6th, 9th, 5th and currently 10th
That's clear regression. It's widely accepted that they've appointed the wrong and sometimes bizarre people in strategic positions and their transfer policy has been poor, while atmosphere within the club has been chaotic and at times toxic.
No one at Nice expected PSG like success, they expected to carry on the progression, there were no expectations of Mbappe or Neymar types suddenly arriving. They're unhappy because the club has been badly let down by the ownership since 2019. Being written off as being upset because they don't have Roman Abramovich like money and success is just wrong and clearly misunderstands fans at lower clubs.
Yes they made a cup final and lost to Nantes, but that only seemed to accelerate the chaos at the club.
I'm not sure why you're defending INEOS so strongly, it doesn't mean they'd muck up at United, maybe they've learnt their lesson as I said the appointment of Florent Ghisolfi is definitely a positive step and if he can replicate his recruitment success at Lens then maybe we'll see better more settled Nice in the future.